The present invention relates to an active material and a process for the production thereof, an electrode and a process for the production thereof, and a non-aqueous electrolyte battery.
With the recent remarkable development of portable electronic apparatus, there has been an urgent demand for the enhancement of the performance of battery used as power supply for these portable electronic apparatus. A secondary lithium battery comprising lithium as a negative active material, which shows the lowest electrode potential in metals and a small specific gravity, has been expected as one of batteries meeting this demand. However, the problem of the battery are low cyclability and poor safety performance because of formation of dendritic lithiums during charge-discharge cycle.
Therefore, a secondary lithium ion battery comprising a carbon material such as graphite and carbon as a negative active material on which dendritic lithium can hardly be deposited, and lithium cobaltate and lithium nickelate as a positive active material has been developed. In recent years, this type of a battery has been used as a high energy density battery.
The positive active material to be used in non-aqueous electrolyte batteries such as lithium ion battery has a low electronic conductivity and thus is applied to a current collector such as aluminum in admixture with an electroconductive material such as acetylene black. A non-aqueous electrolyte battery such as lithium ion battery is composed of various materials such as active material, electroconductive material, binder, current collector and separator.
However, due to the difference in materials, the various materials have drastically different wettability with the electrolyte, making it impossible for the each component of cells to wet uniformly with the electrolyte and hence causing nonuniform current distribution. Therefore, current can be concentrated on local points, deteriorating the high rate discharge performance of the battery.
Further, since the amount of the electrolyte in the battery decreases during cycle life test, the electrolyte on the area having a poor wettability with the electrolyte is absorbed by the area having a good wettability with the electrolyte, making a local insufficiency of the electrolyte and hence deteriorating the cycle life performance of the battery.
Moreover, the foregoing problems make it difficult to decrease the amount of the electrolyte in the battery.
Accordingly, a method of improving the safety by decreasing the amount of a inflammable organic electrolyte cannot be employed.
Further, in the case where a carbon material is used as a negative active material, the electrolyte can be decomposed on the surface of the carbon material during the first charge. irreversible reactions that a film is formed by the decomposed products on the surface of the carbon material occur. Therefore, the reversible capacity of the battery is reduced because of the limitation of the amount of lithium contained in the positive active material.
The secondary lithium ion battery comprising a carbon material as a negative active material is safe as compared with the secondary lithium battery comprising metallic lithium as a negative active material. However, when the temperature of the battery comprising a carbon material was increased by heating from the outside or internal short-circuit, thermal runaway at the positive electrode is occurred by exothermic reaction of lithium-inserted carbon with electrolyte. In order to prevent this trouble, other safety means are provided.
The reaction of the carbon material with the electrolyte occurs on their interface. Accordingly, by coating the surface of the carbon material with a polymer, the irreversible capacity of the negative electrode can be reduced, making it possible to inhibit the exothermic reaction.
For example, techniques have been disclosed which comprise coating the surface of a carbon material capable or lithium insertion and extraction with a polymer electrolyte (JP-A-7-235328 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) or coating the surface of a carbon material with a polymer film made of a polymer and an alkaline metal salt (JP-A-8-306353) to prevent the production of gas from the negative electrode during first charge.
However, even the use of these techniques cannot sufficiently inhibit the deterioration of high rate discharge performance, cyclability and life and the reaction of carbon material, if used in a negative electrode, with the electrolyte.
The present invention is intended to solve the foregoing problems.
The present invention has been worked out on the basis of the discovery that the reason why the foregoing problems cannot be solved by the method involving the coating the surface of active material with a polymer is that the amount of the polymer applied is improper or the method for producing the polymer coating is improper. Further, the present invention has been worked out on the basis of the discovery that when a proper amount of a polymer is used or a proper method of the producing is employed, the form or material of the polymer used can be properly predetermined, making it possible to further improve the performance of the active material.
The composite active material of the present application is provided with a polymer on the surface thereof in an amount of from 0.01% to 5% by weight.
Such a composite active material can be used as the active material for non-aqueous electrolyte battery for example to solve the foregoing problems attributed to wettability with electrolyte.
The polymer content is preferably from 0.1% to 1% by weight. The foregoing composite active material according to the present application preferably acts as a positive active material.
In the case where the active material is a carbon-based active material, the content of the polymer to be provided on the surface thereof is more preferably from 0.04% to 4% by weight. Such a composite active material can be used as the negative active material for non-aqueous electrolyte battery to solve the foregoing problems attributed to the reaction with the electrolyte, not to mention the foregoing problems attributed to wettability.
The term xe2x80x9ccontentxe2x80x9d as used herein is meant to indicate the percentage of the weight of the polymer provided on the surface of the composite active material based on the weight of the composite active material.
The composite active material according to the present application is provided with a polymer on the surface thereof. In this arrangement, by using the polymer as electrolyte and providing the polymer with pores for retaining the electrolyte and forming ion passage, she distribution of the electrolyte on the surface of the active material can be rendered uniform. In the case where the polymer is used as electrolyte or other cases, it is effective to render the polymer porous. This effect is caused by the polymer which enhanced the capability of the composite active material of retaining the electrolyte and facilitated the movement of ions to the active material.
In the composite active material according to the present application, the polymer preferably comprises fluorine incorporated therein whichever structure it has. This is because such a polymer has an excellent durability. The composite active material according to the present application is preferably provided with such a polymer particularly when used as the active material for non-aqueous electrolyte battery.
The process for the production of the composite active material according to the present application comprises providing a polymer solution on the surface of the active material.
In accordance with this process, the surface conditions of the composite active material can be optimized, making it possible to effectively accomplish the function of the polymer. This process can be applied also to the production of an active material having a size as small as molecular level.
This process is suitable for the production of the composite active material according to the present application and is advantageous in that the polymer content can be controlled merely by adjusting the concentration of the solution.
The viscosity of the polymer solution to be used in his process is preferably 20 mPaxc2x7s or less, more preferably 10 mPaxc2x7s or less. The active material is preferably dipped in a polymer solution having a viscosity within the above defined range to provide the polymer solution on the surface of the active material.
Regardless of whatever process is employed, the process for providing the polymer solution on the surface of the active material preferably comprises a first dipping step at which the active material is dipped in the polymer solution in a first pressure atmosphere and a second dipping step at which the active material is dipped in the polymer solution in a second pressure atmosphere having a pressure greater than that of the first pressure atmosphere. The first dipping step is preferably followed by the second dipping step to provide the polymer solution on the surface of the active material.
In the process for the production of the composite active material, the polymer solution provided on the surface of the active material is preferably followed by the phase separation of the polymer from the polymer solution provided on the surface of the active material.
The foregoing phase separation is preferably carried out by dipping the active material provided with the foregoing polymer solution in a non-solvent for the polymer contained in the polymer solution.
The electrode according to the present application comprises a composite active material of the present application or an active material produced by the production process of the present application. The electrode according to the present application is suitable particularly for non-aqueous electrolyte battery.
The electrode according to the present application is preferably provided with a porous polymer on the surface thereof Alternatively, the electrode according to the present application is preferably a porous electrode comprising pores formed between composite active materials, the pores being filled with a porous polymer. Still alternatively, the electrode according to the present application is preferably a porous electrode comprising pores formed between composite active materials having a porous polymer provided both in the pores and on the surface thereof.
The electrode according to the present application can be preferably produced by applying a mixture containing the foregoing composite active material of the present application or the composite active material produced by the process of the present application by coating to a collector.
The non-aqueous electrolyte battery of the present application comprises the composite active material of the present application, the composite active material produced by the process of the present application or the electrode of the present application.
In particular, the non-aqueous electrolyte battery of the present application preferably comprises a composite active material comprising a carbon-based active material provided with a polymer on the surface thereof in an amount of from 0.04% to 4% by weight as a negative active material. Alternatively, the non-aqueous electrolyte battery of the present application preferably comprises a composite active material comprising a positive active material provided with a polymer on the surface thereof in an amount from 0.01% to 5% by weight as a positive active material. Still alternatively, the non aqueous electrolyte battery of the present application preferably comprises both a composite active material comprising a carbon-based active material provided with a polymer on the surface thereof in an amount from 0.04% to 4% by weight as a negative active material and a composite active material comprising a Positive active material provided with a polymer on the surface thereof in an amount of from 0.01% to 5% by weight as a positive active material.
Further, the non-aqueous electrolyte battery of the present application preferably comprises a porous polymer as a separator to exhibit excellent performance.